1. Field of the Invention
The present invention relates to a soldering device or unit and, more particularly, to such a device or unit designed to prevent defects of soldering (hereinafter called the soldering defects) which may otherwise occur when separating the article to be soldered from molten solder flowing out of a nozzle.
2. Statement of the Prior Art
There are known flow type soldering units including a solder bath containing a molten solder material and nozzles adapted to flow out the molten solder material. For instance, such soldering units are widely used when the leads of electrical parts are soldered to soldering lands on the circuit patterns of printed circuit boards (hereinafter referred to as the printed boards) so as to mount the electrical parts on the printed boards.
As illustrated in FIG. 3, one typical example of the flow type soldering units includes a solder bath 1 provided therein with a metallic nozzle 2, out of which molten solder is permitted to flow by means of a built-in impeller. When it is intended to apply solder to one, shown at a, of the aforesaid printed boards using this unit, that board is carried into the flowing molten solder, with its leading end being slightly upward, to come into contact with the solder. Then, the printed board a is successively advanced at the same attitude to allow the molten solder to come into contact with its entire surface.
In order to bring the printed board into satisfactory contact with the molten solder flowing out of the nozzle, the nozzle is provided with wave-rectifying plates 2b and 2c of metal on both sides of its aperture 2a, as viewed in the direction of advancement or forward movement of the printed board, said rectifying plates serving to wave-rectify the flowing molten solder. One type of such wave-rectifying plates is of the so-called oneway-flow structure wherein one wave-rectifying plate 2b, in the direction which the printed board a enters, is arranged with its end inclining downwardly, and the other wave-rectifying plate 2c, in the direction which the printed board a leaves, is constructed from a horizontal piece and a vertical end piece, as illustrated in FIG. 3 as an example. With the wave-rectifying plate combination of this type, the molten solder flows down immediately along the wave-rectifying plate 2b. However, the molten solder does not immediately flow down along the wave-rectifying plate 2c, and is stagnated thereon for a while in the form of an approximately laminar flow. For that reason, in comparison with a nozzle 2' provided on both sides of its aperture with downwardly inclining wave-rectifying plates 2'b and 2'c in FIG. 4, the soldering defects are more difficult to occur, such as "icicles" hanging pieces of solder-occuring when solder on the portion to be soldered separates from a flow of molten solder or "solder bridges" formed by the contact of solder on adjacent portions to be soldered.
Even with the arrangement as mentioned above, however, it is impossible to eliminate completely the occurrence of soldering defects. For instance, one of these causes is that a drop of the temperature of the molten solder stagnating on the wave-rectifying plate 2c for some time is unavoidable for reason of the fact that the molten solder is exposed to the open air for a longer period of time, compared with the molten solder that flows down immediately. In one typical case, there is a temperature drop of as large as 10.degree. C. from 250.degree. C. that is the temperature of the as-flowed molten solder. Under that situation, the printed board -- its one side being applied with a flux and pre-heated to 100.degree. to 140.degree. C. and its other side being at a temperature of 80.degree. to 100.degree. C. -- comes into contact with the solder of 250.degree. C. on the rectifying plate 2b, but only contacts the solder of 240.degree. C., about 10.degree. C. lower temperature on the wave-rectifying plate 2c. At the time when the portion to be soldered separates from the lower temperature molten solder after contacted the higher temperature molten solder, the temperature of the printed board per se is considerably lower than 240.degree. C. so that when solder on the portion to be soldered are detached from a flow of molten solder, their cut ends decrease considerably in temperature and tail off. This may give rise to either the "icicles" or the "solder bridges" due to the contact of adjacent mounted molten solder. Such soldering defects caused by the "icicles" or the "solder bridges" have to be corrected by hand work, and pose a problem in view of working efficiency.